During the electrorefining of copper, impure copper anodes are electrochemically dissolved in an aqueous solution containing copper sulphate and sulphuric acid. At the same time, at the cathode, pure copper is deposited and subsequently recovered from the electrorefining cell. The objective of refining is to make a separation between impurities contained in the anode and final copper cathode product. Some impurities such as gold and silver are retained as a solid product, referred to as "slime", and subsequently physically recovered from the anode residue. Impurities like antimony and bismuth are partly collected in the solid product and partly dissolve into the copper sulphate-sulphuric acid solution.
Small concentrations of antimony and bismuth impurities, as well as others, in the copper sulphate-sulphuric acid solutions can be tolerated up to a certain level. However, if allowed to increase in concentration past refinery specific limits, the copper cathode product will be contaminated with excessive amounts of antimony and bismuth. Additionally iron present in the impure copper anode will dissolve in the solution. Small concentrations of iron in the copper sulphate-sulphuric acid solution do not result in contamination of the copper cathode product. Iron is present in two oxidation states, ferrous (Fe.sup.2+) and ferric (Fe.sup.+). The steady state levels of these impurities in refinery electrolyte vary depending on the composition of the copper anodes being treated and the specific refinery operating conditions. However, typical levels are of the order of 0.2-0.5 g/L for antimony, 0.1-0.6 g/L for bismuth and 0.2-2.0 g/L for iron.
In order to control the build-up of impurity ions in solution, refineries generally employ a purification process to remove the deleterious impurities. The process usually involves a multi-step electrolytic deposition of copper. As copper is depleted, antimony and bismuth, as well as arsenic, begin to co-deposit with the copper product. The contaminated copper product is then recovered and recycled for copper recovery. The solution, after substantial depletion of copper, antimony, arsenic and bismuth may then be subjected to further purification, for example, by evaporative crystallization. The purified solution is then returned to the copper refinery. The disadvantages of the electrolytic purification process are numerous. In particular, the process is energy intensive, an impure copper by-product is produced, and under certain conditions, toxic arsine gas may be evolved.
Alternate methods to replace the conventional electrolytic purification process have been proposed. For example, in the so-called Boliden process disclosed in U.S. Pat. No. 3,753,877, arsenic ion is added to the electrolyte to effect an impurity removal. Unfortunately, the required arsenic addition is detrimental to electrolysis. The Nordeutsche Process described in U.S. Pat. No. 3,696,012 involves contacting the impure copper refinery electrolyte with a B-Stannic acid adsorbent. However, B-Stannic acid is expensive and soluble in the electrolyte. Therefore, this method is not economical because of the excessive loss of reagent.
Solvent extraction processes for antimony and bismuth ions removal have also been reduced to practice. For example, the solvent 2EHAPO.sub.4, a mixture of di-2-ethylhexylphosphoric acid and mono-2-ethylhexyphosphoric acid, can be used to extract both antimony and bismuth out of copper refinery electrolyte. However, the extraction is only possible with low efficiency.
Ion exchange resins have been developed to selectively remove impurities from copper refinery electrolytes. For example, Nagai et al. in U.S. Pat. No. 4,559,216 reports the use of a chelating ion exchange resin possessing imino bis methylene phosphonic acid groups on a phenol resin matrix for the removal of antimony and bismuth from electrolyte. However, this method suffers from the strong extraction of ferric ion from copper refinery electrolyte. The loading of ferric ion tends to limit the capacity of the resin and hence the efficiency of the antimony and bismuth removal process. Ferric ion also elutes more slowly than antimony and bismuth when eluting with 6 mol/L HCl, resulting in a progressive poisoning of the ion exchange material in the long run. The antimony and bismuth containing eluant is also contaminated with large mounts of iron. A further disadvantage of this process is the possible release of excessive amounts of chloride ion from the hydrochloric acid eluant back into the copper electrorefinery circuit. Excessive amounts of chloride interfere with the electrodeposition process and result in a poor quality cathode copper and/or precious metal losses.
Alternate resin products have also been reported to be efficient at antimony and bismuth removal from copper electrolyte. For example, the use of Eporous MX-2 ion exchange resin which contains aminomethylene phosphonic acid functional groups on a polystyrene-divinylbenzene matrix has been reported by Sasaki in Hydrometallurgy and Metallurgy of Copper, 1991, volume III, 245-254. The use of the resin DUOLITE.TM. C-467, which contains aminomethylenephosphonic acid functional groups on a polystyrene divinylbenzene matrix, is also known in the art for impurity removal.
Accordingly, there is a great need to improve methods for the purification of sulphuric acid solution, especially in the field of electrorefining of copper. Such method would allow the selective removal of antimony and bismuth ions avoiding the iron poisoning problems and the chloride associated problems encountered in the operation of the ion exchange method for impurity removal.